Many electronic and electrical systems, such as computer and home entertainment systems, require that electrical power be applied to components of the system according to a particular sequence to avoid causing undue stress and possible damage to the components. Particularly with computer systems, there are many situations in which it is advantageous to delay activation of peripheral devices until after the parent device is powered up and has attained a quiescent state. A typical situation is that of a personal or business computer system where the activation of peripheral devices including a monitor, disk drives and printers, are delayed until after the computer itself is fully on-line. Upon activation of the parent device and after the parent device reaches a quiescent operating state, power can be applied to the peripheral devices. This sequence of powering up a computer system is especially helpful in eliminating undesirable transient currents and random logic states caused by simultaneous power up of the parent and peripheral devices.
For example, in many computer systems, power is first applied to the computer itself before power is applied to the monitor, because the computer supplies the monitor with horizontal and vertical synchronization pulses necessary to prevent the free running of the monitor's horizontal and vertical oscillators. Allowing the oscillators to operate in an unsynchronized condition can result in undue stress to the oscillators and hard failure of the monitor.
Similarly, power is applied to the computer before power is applied to the printer. Otherwise, the printer can potentially back-feed power or control signals to the computer and cause the computer to fail to initialize when the computer subsequently receives power. Consequently, the order and timing of the application of power to and removal of power from certain systems needs to be carefully controlled so as to avoid damaging the system components.
One solution for providing power to systems similar to that described above includes employing an operator to manually turn on the components. Specifically, the operator can power on the computer itself and pause momentarily to allow sufficient time for the computer to reach a quiescent operating state before providing power to the computer's peripheral devices. This method is generally unsatisfactory, because the time delay interval is difficult to control and duplicate manually, and further, because it may be desirable to ensure that the power up and power down of the system always occur according to a particular sequence.
Another solution is to use time delay relays (“TDRs”) to provide a predetermined, fixed time delay between application of power to one component and the next. This method is also unsatisfactory, as well as being very expensive. TDRs are capable only of providing a fixed, or at best, a narrowly adjustable, time delay. Furthermore, the power up delay is typically equal to the power down delay, a condition which may be undesirable in certain cases. Finally, the time delay provided by the TDRs is typically not easy to adjust by an operator.
Therefore, a need exists for an intelligent power distribution system that can provide power up and/or power down sequences and delays for equipment, which overcomes limitations and deficiencies of the prior art.